Longitudinal and vertical gust feed forward compensation using lateral control surfaces

ABSTRACT

In one embodiment of a method to reduce vertical position errors of an aircraft, a disturbance input acting on the aircraft may be determined. The magnitude of the disturbance may be converted into a delta lift command if the magnitude of the disturbance is outside a criteria. The delta lift command may be post processed. The delta lift command may be converted into symmetric lateral surface position commands for control surfaces. The symmetric lateral surface position commands may be communicated to lateral control surface actuators to move the control surfaces according to the symmetric lateral surface position commands.

BACKGROUND

The disclosure relates to aircraft flight control systems andspecifically to the automatic control of an aircraft's flight path.Automatic pilot systems are widely used in the aviation industry toprovide precision guidance to aircraft. Conventional control systemstypically utilize the elevator as the control surface for effectingchanges in the aircraft's vertical path. One objective of the disclosureis to provide wind gust disturbance rejection in order to enhance theprecision of vertical path control afforded by a conventional pitchcontrol systems coupled with an automatic pilot system, both duringlanding and non-landing flight situations.

As a representative example, an automatic landing is a vertical pathtracking task that requires precise vertical path control in order toachieve acceptable performance. Automatic landing capability is requiredfor operations in the most severe low visibility weather, referred to asCategory IIIB low weather minima, and is used in less restrictiveweather minimums to enhance safety and reduce flight crew work load. AnAutomatic Landing System (ALS) provides the precise vertical and lateralpath guidance necessary to meet the stringent performance requirementsspecified for low weather minimum operations.

The vertical path guidance provided by an ALS includes both glide pathcontrol and the flare maneuver. Precise control of vertical positionrelative to the desired vertical path is essential in order to achievethe performance required for Category III operations. The glide pathprovides the established descent gradient and longitudinal positionreference for final approach flight path guidance. The flare maneuverprovides the transition from the glide path to touchdown at the desiredlocation on the runway. Ideally, the ALS will land the aircraft at thesame point on the runway regardless of environment or facility. In otherwords, the design must be very robust given the wide range ofenvironmental conditions, terrain, and runway characteristics that theaircraft will be subjected to during automatic landings. However, inpractice the vertical path tracking provided by the ALS is significantlyaffected by shearing winds, terrain, and runway characteristics. Anyenhancement of an existing autopilot design that improves vertical pathtracking will reduce the impact of the aforementioned disturbancesduring automatic landing operations.

For automatic landings, the autopilot used in airplanes such as the 777,757, 767, and 747-400 utilizes a vertical position control law design toprovide glide path control and the flare maneuver. The elevator commandis generated with an elevator vertical position feedback control system.The vertical position control law design produces a pitch attitudecommand that is proportional to the altitude error and altitude rateerror and the integral of the altitude error. The design is tuned toprovide accurate vertical path tracking with acceptable stabilitycharacteristics. One problem with relying solely on an elevator feedbackcontrol system for vertical position control is that high gains areusually required to achieve the desired vertical path tracking accuracy.However, excessively high gains in the elevator feedback control systemcan compromise the overall system stability, potentially resulting ininteraction with aircraft structural modes. High gains can also resultin the pitch activity that is objectionable to the flight crew.

Autopilots typically utilize a predictive or elevator feed forwardcompensation of some soil in combination with elevator feedback controlto achieve disturbance rejection. This combination of feed forwardcompensation and feedback control allow performance objectives to be metwithout restoring to excessively high and potentially destabilizingfeedback gains. The types of elevator feed forward compensation utilizedare typically either short term moment compensation or long term forcecompensation.

For short term moment compensation, a control surface command (elevator)is generated such that a moment is created that cancels the momentpredicted to be generated by the disturbance. For long term forcecompensation, a pitch attitude command is generated to counteract thesteady state trim changes due to a disturbance. Short term momentcompensation tends to limit pitch attitude change in response to adisturbance, whereas long term force compensation tends to generatepitch attitude change in response to a disturbance. Short term momentcompensation is used for balancing pitching moments due to changes instabilizer, and changes in thrust and ground effects, but is not veryeffective for dealing with vertical path disturbance due to changingwinds. Long term for compensation, on the other hand, is quite effectivein countering the disturbances due to changing winds. However, duringthe flare maneuver, the pitch attitude changes resulting from long termforce compensation tend to result in undesirable pitch activity from afight crew acceptability standpoint.

During a landing there are also geometrical constraints that need to beconsidered. The pitch attitude of the airplane must be limited toprevent ground contact of the nose landing gear prior to the mainlanding gear and ground contact of the aft body (tail strike). Whilelimiting the pitch attitude within the geometrical constraints reducesthe probability of a nose gear first contact and tail strike during anautomatic landing, the ability of the autopilot to maintain thecommanded vertical path can be diminished by these geometricalconstraints. For example, during a flare maneuver, the geometricalconstraints may prevent the autopilot from maneuvering as aggressivelyin response to the vertical path upset caused by shearing winds.

A method for reducing vertical position errors of an aircraft is neededto decrease one or more problems associated with one or more of theexisting methods.

SUMMARY

In one aspect of the disclosure a method is disclosed for reducingvertical position errors of an aircraft. In one step, a disturbanceinput acting on the aircraft may be determined. In another step, adetermination may be made as to whether the magnitude of the disturbanceexceeds a criteria. In still another step, no more steps of the methodmay be followed if the magnitude of the disturbance is not outside thecriteria. The magnitude of the disturbance may be converted into a deltalift command if the magnitude of the disturbance is outside thecriteria. In yet another step, the delta lift command may be postprocessed. In an additional step, the delta lift command may beconverted into symmetric lateral surface position commands for controlsurfaces. In another step, the symmetric lateral surface positioncommands may be communicated to lateral control surface actuators tomove the control surfaces according to the symmetric lateral surfaceposition commands.

In another aspect of the disclosure, a method is disclosed for reducingvertical position errors of an aircraft due to wind gusts. In one step,a magnitude of a vertical wind gust acting on the aircraft may bedetermined using an angle of attack rate and a magnitude of alongitudinal wind gust acting on the aircraft may be determined using atrue airspeed rate. In an additional step, a vertical wind gust signaland a longitudinal wind gust signal may be passed through a criteria,and no more steps of the method may be followed if the vertical windgust signal and the longitudinal wind gust signal are not outside thecriteria. In another step, the vertical wind gust signal and thelongitudinal wind gust signal may be multiplied by at least one gainsignal to produce two delta lift commands. In an additional step, thetwo delta lift commands may be summed to produce a single delta liftcommand. In yet another step, the single delta lift command may be usedin unison with a conventional vertical position feedback elevatorcontrol loop to compensate for wind gusts in order to maintain acommanded position of the aircraft.

These and other features, aspects and advantages of the disclosure willbecome better understood with reference to the following drawings,description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system block diagram which may be used under oneembodiment of the disclosure;

FIG. 2 shows a top view of one embodiment of control surfaces of anaircraft;

FIG. 3 shows a flowchart of one embodiment of a method for reducingvertical position errors in an aircraft; and

FIG. 4 shows one embodiment of a block diagram which may be followed toimplement the method of FIG. 3.

DETAILED DESCRIPTION

The following detailed description is of the best currently contemplatedmodes of carrying out the disclosure. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the disclosure, since the scope of thedisclosure is best defined by the appended claims.

FIG. 1 shows a system block diagram 10 which may be used under oneembodiment of the disclosure. The system 10 may include one or moreangle of attack sensor 12, one or more true airspeed sensor 14, anautopilot system 16, a flight control 18, one or more actuator 20, oneor more control surface 22, and one or more computer 24.

The angle of attack sensor 12 may be adapted to sense an angle of attackof an aircraft. The true airspeed sensor 14 may be adapted to sense anairspeed of an aircraft. The autopilot system 16 may comprise an angleof attack rate calculator, a true airspeed rate calculator, and/or othertypes of autopilot devices. The flight control system 18 may compriseone or more of a delta lift to surface deflection converter, a surfacecommand processor, and/or other types of flight controls. The one ormore actuators 20 may comprise one or more devices that may be used tomove the control surfaces 22. The one or more computers 24 may comprisea feed forward compensation symmetric lateral control surface deflection(or direct lift) computer. The one or more computers 24 may furthercomprise one or more of a processor, a memory, an autopilot interfacemodule, a flight control interface module, and/or other types ofcomputer systems. In other embodiments, the system 10 may includevarying sensors, systems, and/or devices.

As shown in FIG. 2, which shows a top view of one embodiment of controlsurfaces 22 of an aircraft 23, the control surfaces 22 may comprisespoilers 25, ailerons 27, flaperons 29, an elevator 31, and/or othertypes of control surfaces. The spoilers 25 may be deflectedasymmetrically for lateral control, and/or can be symmetricallydeflected for longitudinal control, and/or lift reduction. The ailerons27 may be deflected asymmetrically for lateral control, and/or may besymmetrically deflected for longitudinal control. The flaperons 29 maybe deflected asymmetrically for lateral control, and/or may besymmetrically deflected for longitudinal control and/or lift generation.The elevator 31 may comprise a longitudinal control surface.

FIG. 3 shows a flowchart of one embodiment of a method 32 for reducingvertical position errors in the aircraft 23 of FIG. 2. FIG. 4 shows oneembodiment of a block diagram 34 which may be followed to implement themethod 32 of FIG. 3. As shown in FIGS. 1 and 3, in step 36 a disturbanceinput, such as longitudinal and/or vertical wind gusts acting on theaircraft 23, may be determined. This determination may be done usingaircraft sensors which may comprise the angle of attack sensor 12, thetrue airspeed sensor 14, and/or other type of aircraft sensors. Theaircraft sensors may be used to estimate the longitudinal and verticalwind gusts or other disturbances acting on the aircraft 23 using thecomputer 24, the autopilot system 16, the flight control system 18,and/or signal processing.

Step 36 may comprise, as shown in FIGS. 1 and 4, determining thevertical gust 38 using the angle of attack rate as determined by theangle of attack sensor 12, and/or determining the longitudinal gust 40using the true airspeed rate as determined by the true airspeed sensor14. The autopilot system 16, flight control system 18, and/or computer24 may be used to make this determination.

As shown in FIGS. 1 and 3, in step 42 a determination may be made as towhether the magnitude of the disturbance is outside a criteria, andtherefore sufficiently large to warrant a deflection of the lateralcontrol surfaces 22. This determination may be made using the computer24, the flight control system 18, and/or the autopilot system 16.Factors such as actuator wear, surface fatigue, system stability, and/orother factors may be considered in setting the criteria, which maycomprise a deadzone, a deadzone filter, a filter, and/or other type ofcriteria. Step 42 may comprise, as shown in FIGS. 1 and 4,sending/passing the vertical gust signal 38 through deadzone 44 and/orthe longitudinal gust signal 40 through deadzone 46 using the autopilotsystem 16, the flight control system 18, and/or the computer 24.

As shown in FIG. 3, if the magnitude of the disturbance is not outsidethe criteria, then in step 48 the method may conclude/end 50 withoutdoing/completing any more steps of the method 32. If the magnitude ofthe disturbance is outside the criteria, as shown in step 51, then instep 52 the magnitude of the disturbance may be converted into a deltalift command using the autopilot system 16, the flight control system18, and/or the computer 24 of FIG. 1. Step 52 may comprise, as shown inFIG. 4, using the autopilot system 16, the flight control system 18,and/or the computer 24 to multiply the vertical gust signal 38 by afirst gain signal 54 to produce a first delta lift command 56, tomultiply the longitudinal gust signal 40 by a second gain signal 58 toproduce a second delta lift command 60, and to sum the first delta liftcommand 56 and the second delta lift command 60 to obtain the delta liftcommand 62. The delta lift command 62 may be proportional to themagnitude of the disturbance. The first and second gain signals 54 and58 may be identical. In other embodiments, the first and second gainsignals 54 and 58 may vary.

As shown in FIG. 3, in step 64 the delta lift command 62 may be postprocessed to prevent command saturation. The post processing maycomprise limiting, filtering, and/or smoothing of the delta lift command62. The post processing may be done utilizing at least one of theautopilot system 16, the flight control system 18, and/or the computer24 of FIG. 1. Step 64 may comprise, as shown in FIG. 4,filtering/smoothing/and/or limiting 66 the delta lift command 62 usingthe flight control system 18, autopilot system 16, and/or computer 24.This limiting/filtering/and/or smoothing may prevent commanding moredelta lift than is available with the applicable lateral controlsurfaces 22 shown in FIG. 2. If any of the input signals areexceptionally noisy, then appropriate filtering of the delta liftcoefficient command 62 may be applied. Other sorts of limiting,smoothing, and/or filtering may also be applied as appropriate.

As shown in FIGS. 1-4, in step 68 the delta lift command 62, which mayhave been smoothed/filtered/and/or limited in step 64, may then beconverted into symmetric lateral surface position commands for thecontrol surfaces 22. This may be done using the computer 24, theautopilot system 16, and/or the flight control system 18. The symmetriclateral surface position commands may comprise symmetric lateral controlsurface commands for the control surfaces 22. Step 68 may comprise, asshown in FIG. 4, converting the delta lift command 62 (which may havebeen limited/filtered/and/or smoothed) into symmetric lateral surfaceposition commands 70 using the flight control system 18, the autopilotsystem 16, and/or the computer 24.

As shown in FIGS. 1, 2, and 3, in step 72, the symmetric lateral surfaceposition commands may be communicated to the lateral control surfaceactuators 20 which may control/move the control surfaces 22 according tothe lateral surface position commands. This may be done using the flightcontrol system 18, the autopilot system 16, and/or the computer 24. Thesymmetric deflections of the control surfaces 22 may create a change inlift of the aircraft 23 which is proportional to the disturbance, suchthat the effect of the disturbance is reduced or canceled. The feedbackcontrol loop may work in parallel/unison with the conventional verticalposition feedback elevator control loop (the elevator command) tomaintain the commanded vertical path of the aircraft 23 and tocorrespondingly reduce vertical position error. The method 32 may thenend 50. Step 72, as shown in FIGS. 1 and 4, may comprise communicating74 the symmetric lateral surface position commands to the lateralcontrol surface actuators 20 to control/move the control surfaces 22using the flight control system 18, the autopilot system 16, and/or thecomputer 24.

The embodiments of the disclosure may be used to enhance the accuracy ofthe automatic pilot vertical position command tracking task provided byone or more of the conventional longitudinal control systems. Theenhancement may be achieved by using a feed forward compensator(s) toproduce commands that may result in symmetric deflections of lateralcontrol surfaces on the aircraft's wings that are proportional to thelongitudinal and/or vertical wind gusts. Symmetric deflections of thewing's lateral control surfaces may result in small changes in lift tocounter the vertical path disturbance caused by the gusts. In suchmanner, vertical position command tracking may be improved during anautomatic landing. However, the embodiments of the disclosure could beapplied to any phase of flight where a vertical position controlstrategy is utilized.

For automatic landings, the improved vertical position command trackingachieved by one or more embodiments of the disclosure may increase therobustness and improve the performance of an existing automatic landingsystem. Symmetric deflections of the lateral control surfaces mayproduce significantly less pitching movement than elevator deflections.Therefore, one or more embodiments of the disclosure may provide aunique way to improve vertical position command tracking during anautomatic landing without creating pitch activity that may beobjectionable to the flight crew or requiring excessively high verticalposition feedback gains that may compromise system stability. Thedevelopment and certification of an automatic landing system may be acostly endeavor, requiring extensive flight testing, gain tuning, andsimulation model updates. A more robust automatic landing system may beless sensitive to discrepancies between the simulation models foraerodynamic and sensors and the actual aircraft aerodynamics and sensorcharacteristics, and may therefore reduce the overall cost and designrefinement involved in certification of the automatic landing system.Additionally, lower vertical position feedback gains may reduce thepossibility of structural mode interaction.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the disclosure and that modifications may bemade without departing from the spirit and scope of the disclosure asset forth in the following claims.

1. A method for reducing vertical position errors of an aircraftcomprising the steps of: determining a disturbance input acting on theaircraft; determining whether a magnitude of the disturbance exceedscriteria; following no more steps of the method if the magnitude of thedisturbance is not outside the criteria) and converting the magnitude ofthe disturbance into a delta lift command if the magnitude of thedisturbance is outside the criteria; post processing the delta liftcommand; converting the delta lift command into symmetric lateralsurface position commands for control surfaces; and communicating thesymmetric lateral surface position commands to lateral control surfaceactuators to move the control surfaces according to the symmetriclateral surface position commands.
 2. The method of claim 1 wherein thedisturbance input comprises at least one of a longitudinal wind gust anda vertical wind gust acting on the aircraft.
 3. The method of claim 1wherein the disturbance input is determined using at least one ofaircraft sensors, a computer, and signal processing.
 4. The method ofclaim 2 wherein at least one of the longitudinal wind gust is determinedusing a true airspeed rate and the vertical wind gust is determinedusing an angle of attack rate.
 5. The method of claim 4 wherein at leastone of the true airspeed rate is determined using a true airspeed sensorand the angle of attack rate is determined using an angle of attacksensor.
 6. The method of claim 1 wherein the step of determining whetherthe magnitude of the disturbance is outside the criteria comprisessending at least one signal through at least one deadzone using at leastone of an autopilot system, a flight control system, and a computer. 7.The method of claim 6 wherein the step of determining whether themagnitude of the disturbance is outside the at least one deadzonecomprises sending a vertical gust signal through the at least onedeadzone and sending a longitudinal gust signal through the at least onedeadzone.
 8. The method of claim 1 wherein the step of determiningwhether the magnitude of the disturbance is outside the criteriautilizes at least one of a computer, all autopilot system, and a flightcontrol system.
 9. The method of claim 1 further comprising the step ofsetting the criteria utilizing at least one of actuator wear, surfacefatigue, and system stability.
 10. The method of claim 1 wherein thestep of converting the magnitude of the disturbance into the delta liftcommand utilizes at least one of a computer, an autopilot system, and aflight control system.
 11. The method of claim 1 wherein the step ofconverting the magnitude of the disturbance into the delta lift commandutilizes at least one of an autopilot system, a flight control system,and a computer to multiply a vertical gust signal by a first gain signalto produce a first delta lift command, to multiply a longitudinal gustsignal by a second gain signal to produce a second delta lift command,and to sum the first delta lift command and the second delta liftcommand to obtain the delta lift command.
 12. The method of claim 1wherein the delta lift command is proportional to the magnitude of thedisturbance.
 13. The method of claim 11 wherein the first and secondgain signals are identical.
 14. The method of claim 1 wherein the postprocessing step comprises at least one of limiting, filtering, andsmoothing the delta lift command using at least one of an autopilotsystem, a flight control system, and a computer.
 15. The method of claim1 further comprising the step of filtering the delta lift command if anyinput signals are noisy.
 16. The method of claim 1 wherein the step ofconverting the delta lift command into the symmetric lateral surfaceposition commands for control surfaces uses at least one of a flightcontrol system, an autopilot system, and a computer.
 17. The method ofclaim 1 wherein the control surfaces comprise at least one of a spoiler,a flaperon, an aileron, and an elevator.
 18. The method of claim 1wherein the step of communicating the symmetric lateral surface positioncommands to the lateral control surface actuators to move the controlsurfaces according to the symmetric lateral surface commands uses atleast one of a flight control system, an autopilot system, and acomputer.
 19. The method of claim 1 wherein a feedback control loopworks in unison with a conventional vertical position feedback elevatorcontrol loop to maintain a commanded vertical path of the aircraft andto reduce vertical position error.
 20. The method of claim 1 wherein thesymmetric movement of the control surfaces creates a change in lift ofthe aircraft which is proportional to the disturbance to at least one ofreduce and cancel the disturbance.
 21. A method for reducing verticalposition errors of an aircraft due to wind gusts comprising the steps ofdetermining a magnitude of a vertical wind gust acting on the aircraftusing an angle of attack rate and a magnitude of a longitudinal windgust acting on the aircraft using a true airspeed rate; passing avertical wind gust signal and a longitudinal wind gust signal through acriteria and following no more steps of the method if the vertical windgust signal and the longitudinal wind gust signal are not outside thecriteria; multiplying the vertical wind gust signal and the longitudinalwind gust signal by at least one gain signal to produce two delta liftcommands; summing the two delta lift commands to produce a single deltalift command; and using the single delta lift command in unison with aconventional vertical position feedback elevator control loop tocompensate for wind gusts in order to maintain a commanded position ofthe aircraft.
 22. The method of claim 21 further comprising the step ofat least one of limiting, filtering, and smoothing the delta liftcommand using at least one of a flight control system, an autopilotsystem, and a computer.
 23. The method of claim 21 further comprisingthe step of converting the single delta lift command into symmetriclateral surface position commands for control surfaces using at leastone of a flight control system, an autopilot system, and a computer. 24.The method of claim 23 further comprising the step of communicating thesymmetric lateral surface position commands to lateral control surfaceactuators to move the control surfaces according to the symmetriclateral surface commands using at least one of a flight control system,an autopilot system, and a computer.
 25. The method of claim 24 whereinthe criteria comprise at least one of a deadzone and a filter.